Laurie G. Smith
Assistant Professor of Biology, UCSD

e-mail: lsmith@biomail.ucsd.edu

  Plant tissues are composed of a variety of distinct cell types arranged in a configuration that enables each tissue to carry out its function. We are interested in processes that establish the cellular architecture of plant tissues during development. Because the shapes and positions of plant cells are defined by their walls, the cellular architecture of plant tissues reflects both the pattern of cell division - where new walls are positioned at the conclusion of cytokinesis - and patterns of cell expansion. We are taking a molecular genetic approach to study these processes. We have taken advantage of the extremely regular cellular organization of the maize leaf epidermis by isolating several mutants that perturb this pattern (see images, below). Analysis of these mutant phenotypes has shown that these mutants define genes required for the establishment of cell polarity, the spatial control of cytokinesis, and cell morphogenesis. Some of these genes have been cloned, and we continue to study the functions of these genes and their protein products in both maize and Arabidopsis.


CELL DIVISION
tangled, discordia and pangloss mutants show irregular cellular organization in the leaf epidermis (see illustrations). Analysis of the tangled phenotype has shown that this gene is required for the spatial control of cytokinesis in all organs and cell types that have been examined. tangled mutant cells show defects both in selection of the division plane early in the cell cycle and guidance of the phragmoplast (the cytokinetic apparatus of plant cells) to the appropriate division plane during cytokinesis. In contrast to tangled, discordia and pangloss mutations perturb only the asymmetric divisions that occur relatively late in leaf development to create various specialized cell types. Three different Pangloss genes are required for the establishment of polarity in asymmetrically dividing cells, whereas three Discordia genes are required later for guidance of the phragmoplast to the asymmetric division plane. The nature of Discordia and Pangloss gene products is not yet known; cloning these genes is one of our highest priorities.
The Tangled gene encodes a highly basic protein that can bind directly to microtubules and belongs to a family of proteins that are preferentially associated with the cytoskeletal structures in dividing cells that become misoriented in tangled mutants. We continue to investigate how TANGLED acts to orient cytoskeletal structures in dividing cells. We have recently extended these studies into Arabidopsis through analysis of an Arabidopsis Tangled-like gene, ATN.

CELL MORPHOGENESIS
Leaf epidermal cells in most plant species have lobed shapes; lobes of adjacent cells interlock, probably to increase the mechanical strength of the epidermis as a cell sheet. Three different Brick (Brk) genes acting in a single pathway are required for the formation of epidermal cell lobes. In wild-type cells, dense patches of cortical actin mark the sites where lobes emerge, and persist at the lobe tips as they grow out. In brk mutants, these cortical actin patches fail to form. These observations suggest that local actin polymerization is critical for lobe formation. While the Brk2 and Brk3 genes have not yet been cloned, we have shown that Brk1 encodes a small, highly conserved protein that has recently been implicated in stimulation of actin polymerization via regulation of the Arp2,3 activator, WAVE, in mammalian cells. We continue to study the function and localization of the BRK1 protein in maize and Arabidopsis.

     Smith, L.G., Gerttula, S., Han, S., and Levy, J. (2001).  TANGLED1: A microtubule binding protein required for spatial control of cytokinesis in maize. J. Cell Biol. 152:231-236.

     Frank, M.J., and Smith, L.G. (2002).  A small, novel protein highly conserved in plants and animals promotes the polarized growth and division of maize leaf epidermal cells. Curr. Biol 12:849-853.

     Frank, M., Egile, C., Dyachok, J., Djakovic, S., Nolasco, M., Li, R., and Smith, L.G. (2004). Activation of Arp2/3 complex-dependent actin polymerization by plant proteins distantly related to Scar/WAVE. Proc. Nat. Acad. Sci. USA 101:16379-16384.

     Smith, L.G. and Oppenheimer, D.G. (2005). Spatial control of cell expansion by the plant cytoskeleton. Annu. Rev. Cell Dev. Biol. 21:271-295.

     Djakovic, S.N., Dyachok, J., Burke, M.P., Frank, M.J., and Smith, L.G. (2006). BRICK1/HSPC300 acts with SCAR and the ARP2/3 complex to regulate epidermal cell shape in Arabidopsis. Development 133:1091-1100.

     Muller, S., Han, S., and Smith, L.G. (2006). Two kinesins are involved in the spatial control of cytokinesis in Arabidopsis thaliana. Curr. Biol., 16:888-894.
After receiving her Ph.D. from University College, London, Laurie Smith conducted postdoctoral study at Stanford University and the UC Berkeley/USDA Plant Gene Expression Center.